1. Field of the Invention
The invention relates generally to a method of vacuum ultraviolet lithography, and in particular, to a method of lithography in which the irradiating wavelength is selected to be in a region of low absorption by air.
2. Description of the Background Art
The fabrication of microelectronics devices typically involves a complicated process sequence requiring hundreds of individual steps performed on semiconductive, dielectric and conductive substrates. Examples of these process steps include oxidation, diffusion, ion implantation, thin film deposition, cleaning, etching and lithography. Lithography and etching are often referred to as pattern transfer stepsxe2x80x94a circuit pattern is first transferred to a photosensitive material layer using lithography, and then to the underlying material layer during the subsequent etching step. These processes are well known to one skilled in the art, and details of conventional lithography can be found, for example, in xe2x80x9cIntroduction to Lithographyxe2x80x9d by Thompson et al. (ACS 1983) and xe2x80x9cSemiconductor Lithographyxe2x80x9d by Moreau (Plenum Press 1988). Processes related to the fabrication of integrated circuits are discussed in Silicon Processing for the VLSI Era, by Wolf and Tauber (Lattice Press 1986). Each of these references is herein incorporated by reference.
FIG. 1 is a schematic diagram illustrating the basic principle of a lithographic system used in the fabrication of integrated circuits (IC). A source of radiation 110 exposes a radiation sensitive material 160, known as a resist, that has been coated on the substrate surface 180. The resist 160 is typically a polymeric material which undergoes structural or chemical changes upon exposure to the incident radiation. The incident radiation, which is typically of narrow bandwidth, is either provided by conventional light sources such as a mercury lamp, or excimer laser systems such as krypton fluoride (KrF) or argon fluoride (ArF). For example, the bandwidth of the incident radiation may be controlled by the use of an appropriate filter 115. A mask 130, or reticle, is positioned between the light source 110 and the substrate 180 containing the resist layer 160. A typical optical imaging system for photolithography comprises a lens component 120 used to collimate the light, or radiation, to illuminate the mask 130. The light which is transmitted through the mask 130 is subsequently focused by additional imaging optics 140 onto the resist layer 160. The mask 130 contains regions such as 130a that transmit the radiation and regions 130b that block the radiation. A typical mask, for example, consists of a chrome pattern (corresponding to a circuit pattern) that has been formed over a quartz substrate 132. While quartz is transparent to the incident radiation, the chrome pattern prevents the radiation from reaching the resist layer 160. For example, this lithographic exposure step may render the exposed resist region soluble to a chemical in a subsequent developing step, and allows the circuit pattern to be transferred to the resist layer. This pattern transfer process from the mask to the resist layer 160 is analogous to exposing printing paper through a negative in conventional photography. Of course, in modern IC fabrication methods, the dimensions of the patterned features are typically smaller than 0.5 xcexcm in width, and often considerably lessxe2x80x94around 0.25 xcexcm for state of the art devices.
Efforts are ongoing to develop lithographic systems with increasing resolution in order to meet the demand for faster IC with a higher density of circuitry. This, in turn, has created the need for shorter wavelength radiation sources for lithographic applications, since the resolution of an optical imaging system is directly proportional to the source, or irradiation wavelength. The resolution R of a lithographic imaging system is given by R=kxcex/N.A., where k is a proportionality constant which is process-dependent, xcex is the incident wavelength (sometimes referred to as the irradiating, or xe2x80x9cactinicxe2x80x9d wavelength), and N.A. is the numerical aperture of the imaging optics.
A variety of radiation sources in the visible, ultraviolet (UV), deep ultraviolet (DUV), and X-ray regions of the electromagnetic spectrum are currently used in IC manufacturing or under development. Details relating to these lithographic methods, their advantages and disadvantages, are well known to those skilled in the art. For example, recent commercial lithographic systems have employed exposure or irradiating wavelengths at 436 nm (atomic mercury xe2x80x9cGxe2x80x9d line), 365 nm (atomic mercury xe2x80x9cIxe2x80x9d line), 248 nm (KrF) and 193 nm (ArF). While mercury lamps are used as radiation sources for the xe2x80x9cGxe2x80x9d and xe2x80x9cIxe2x80x9d lines, pulsed light sources such as rare-gas halide lasers are used to produce radiation at 248 nm and 193 nm for deep ultraviolet (DUV) lithography. Even though the development of production worthy lithographic systems with laser sources is extremely challenging, the basic physics of the lithographic process is well known.
The trend towards developing shorter wavelength systems primarily involves a migration to laser radiation sources. There are many drawbacks, however, to existing laser systems such as shot-to-shot intensity variation, long-term output intensity drift, need for line narrowing, and high maintenance cost. The need for ultra-high purity gases, additional safety requirement for toxic gas handling, coupled with the typically large footprint of the laser system, all add considerable complexities to their adaptation in a cleanroom environment. The cost of the laser light source, which may be as much as $500,000, also contributes significantly to the overall capital cost for such a lithographic tool.
As lithographic development extends into the vacuum ultraviolet (VUV) region, corresponding approximately to wavelength below 1900 xc3x85, another level of complexity arises due to the absorption of short wavelength radiation, which stems primarily from the oxygen present in air. To a first approximation, the absorption of incident radiation by a gaseous medium increases with increasing absorption coefficient, itself dependent upon the wavelength of radiation, the pressure and the path length of the medium. A higher transmitted radiation intensity can be obtained by either working in a non-absorbing medium, or reducing the operating pressure and/or the path length. The path length of a typical lithographic system (for the present discussion, is roughly the distance between the radiation source and the substrate being processed), is primarily dictated by the optical design requirements of a specific lithographic tool. Therefore, in practice, the operation of a VUV lithographic system will involve either inert gas purging or evacuation of the beam path to provide an environment with a reduced oxygen content.
Recently, the fluorine (F2) laser emission at 157 nm has been used for VUV projection patterning experiments. Rothschild et al. (Lambda-Physik publication, July 1998) reported printing 80 nm lines using a simple projection system, a commercial F2 laser source, a chromeless phase shifting mask made of calcium fluoride and top surface imaging resist processing. To overcome oxygen and water vapor absorption, which would dramatically reduce the beam intensity available for resist exposure, the beam path was purged with dry nitrogen. This reference is herein incorporated by reference.
An alternative to gas purging would be evacuation of the beam path. In any case, this vacuum or purging requirement translates to additional processing time for a fabrication step which is already recognized as the throughput xe2x80x9cbottleneckxe2x80x9d in the IC manufacture process, because unlike most process steps, lithography cannot be performed as a batch process. Therefore, a need exists in the art for alternative light sources or methods of lithography in the VUV region, that can give the requisite submicron pattern resolution, while avoiding lengthy pump-down or purging cycle times.
The present invention is a method of performing vacuum ultraviolet (VUV) lithography by selecting an irradiating wavelength corresponding to a region of low absorption by airxe2x80x94e.g., in the vicinity of a local minimum in the oxygen absorption spectrum. This method of lithography relaxes the stringent requirement on vacuum or inert atmosphere that is otherwise imposed on the VUV lithographic tool.
In one embodiment, VUV lithography is performed at a wavelength between about 121.0 and 122.0 nm, and most preferably, at about 121.6 nm, such as that available from an atomic hydrogen transition known as the hydrogen Lyman-xcex1 emission line. This particular wavelength corresponds closely to a minimum in the oxygen absorption spectrum. The present invention offers the benefit of a less complex lithographic tool by dramatically relaxing the otherwise stringent reduced oxygen requirement. A hydrogen discharge lamp, for example, is a convenient and relatively inexpensive radiation source for generating the hydrogen Lyman-xcex1 emission line for practicing this invention.